1. The crystals of the complex of an anti-lysozyme Fab and lysozyme have been grown to a size suitable for X-ray diffraction studies. X-ray data have been collected at the Cornell High Energy Synchrotron Source. 2. The phosphocholine-binding mouse myeloma Fab crystal structure has been refined completely. The final structure has been used to examine the molecular nature of the phosphocholine binding site. 3. The structure of the galactan binding mouse myeloma protein J539 has been refined at 2.8 Angstrom resolution. The structure is now being used to define the hapten combining site.